1. Field of the Invention
This invention relates to a combined magnetic head core and a manufacturing method therefor and more particularly to a combined magnetic head core made up of a pair of a record/reproduce cores and an erase core for use with a device, such as a floppy disk drive, and a manufacturing method therefor.
2. Description of the Related Art
Hitherto, a ferrite core formed using ferrite as core material has been known as a magnetic head core. The ferrite core generally has a ring form made up of a pair of ferrite members, whereby a ring magnetic path (magnetic circuit) is formed. A predetermined magnetic gap is provided between the opposite ferrite members in the direction crossing the magnetic path. By the use of a magnetic gap, predetermined recording and reproducing are performed on magnetic recording media such as a magnetic disk by bringing the outer face of the ferrite members, between which the magnetic gap is formed, into contact with the magnetic recording medium. A space is formed between the paired ferrite members and is used as a hole for coil winding.
In addition to using such a single ferrite core as a magnetic head core, there may be a combined magnetic head core into which the two ferrite cores are integrated, one example of which is shown in Japanese Patent Laid-Open Nos. Hei 2-47002 and 3-11002. FIG. 1 is a perspective view of a combined magnetic head having a conventional combined magnetic head core structure. FIG. 2 is a perspective view of the conventional combined magnetic head core structure.
The conventional combined magnetic head 18 consists mainly of a combined magnetic head core 10, a slider 16, two coil assemblies (not shown), and two back cores (not shown). The combined magnetic head core 10 comprises a record/reproduce core (read/write core: R/W core) 12 and an erase core (E core) 14. The R/W core 12 has an outer core section 20 and an inner core section 22. The outer core section 20 is made of a ferrite member whose side is formed like an L letter; it consists of a contact area 20a with which magnetic recording media are brought into contact in the upper right of FIG. 2 and a coil core 20b inserted into an insertion hole of the slider 16. The inner core section 22 is made of a ferrite member whose side is formed like a letter "I" letter; it has a contact area 22a with which magnetic recording media are brought into contact in the upper center of FIG. 2, like the outer core section 20. The outer core section 20 and the inner core section 22 form a magnetic gap (read/write gap) 30 between the contact areas 20a and 22a. A read/write track 32 for recording and reproducing data on and from magnetic recording media is formed in the contact areas 20a and 22a.
The read/write track 32 is determined so that it has a predetermined width (w) according to the type of magnetic recording medium with which it is brought into contact. The track width is defined by track definition grooves 34 formed in both magnetic path sides of the magnetic gap 30, namely, formed in both sides of the magnetic path in the axial direction of the contact areas 20a and 22a. The track definition grooves 34 are tilt grooves formed across both the contact areas 20a and 22a.
The erase core 14 has an outer core section 36 and an inner core section 38. The outer core section 36 is made of a ferrite member whose side is formed like a letter "L"; it consists of a contact area 36a and a coil core 36b. The inner core section 38 is made of a ferrite member whose side is formed like a letter "I"; it has a contact area 38a with which magnetic recording media are brought into contact in the upper center of FIG. 2. The outer core section 36 and the inner core section 38 form two magnetic gaps (erase gaps) 40 between the contact areas 36a and 38a. Tilt track definition grooves 42a and 42b are formed in the surface of the contact areas 36a and 38a, across the contact areas 36a and 38a, in the center and in both sides of the magnetic path of the contact areas 36a and 38a. The magnetic path of the contact area 36a formed in the left and right of the track definition groove 42a forms erase tracks 44.
The inner core sections 22 and 38 of the record/reproduce core 12 and the erase core 14 are located back to back with a predetermined gap. The gap between the inner core sections 22 and 38, the track definition grooves 34, 42a, and 42b, and the magnetic gaps 30 and 40 are filled with glass 46 to protect the read/write track 32 and the erase track 44.
A method of manufacturing the conventional combined magnetic head core comprises the steps of "surface groove processing of a first core member (U bar blank) and deposition of a second core member (I bar blank)," "first glass bonding (first GB)," "first GB finishing," "track definition groove processing on record/reproduce core," "track definition groove processing on erase core," "second glass bonding (second GB)," "second GB top processing," "coil groove processing," and "core cutting and core chip preparation."
The method is described in conjunction with FIGS. 3 and 4. First, in FIG. 3, with a surface 50a on the side of incomplete dimension (pole height (PH)) of magnetic gap depth of a U bar 50, a first core member made of ferrite, etc., whose section perpendicular to the length of it is formed substantially like a letter U as processing reference, an opposite face 52 is processed and the full width of U bar blank is set. The PH dimension of the U bar 50 is defined by the magnetic gap formation surface and the apex (AP), and the back height (BH) dimension is defined by the AP and the bottom of coil core 50b. Generally, the U bar 50 and an I bar 54, a second core member made of ferrite, etc., whose section perpendicular to the length of it is formed substantially like a letter I are set as having the same width (W in FIG. 3). With, as reference, the surface 50a on the PH side of a combined core structure, into which two core blanks subjected to first glass bonding by aligning the surface 50a on the PH side of the U bar 50 and one face of the I bar 54 are integrated by second glass bonding, the opposite face 52 to the PH side is processed and the combined core structure is cut to a predetermined thickness to provide combined core chips 56. This BH becomes complete BH. Therefore, for the U bar 50, high precision is required for the PH although the BH at the blank time may be low in precision.
Next, in FIG. 4, when the combined core chips 56 are bonded to the sliders 16, if faces 16a on the magnetic recording medium contact side of the sliders 16 are aligned on the side reference of the PH side of the combined core chips 56 for bonding, the lower faces 16b of the sliders 16 and the surfaces 10 on the BH side of the core chips 56 do not align due to the dimension difference between the sliders 16 and the core chips 56. To process the faces 16a on the magnetic recording medium contact side of the sliders 16 as shown in FIG. 4, even if the PHs of the core chips 56 are precise, if the heights of the sliders 16 vary in precision, the height difference B between the sliders 16 occurs. Since a plurality of pieces are processed at the same time with the lower faces 16b of the sliders 16 as reference, gap depth (GD) precision becomes bad. Therefore, height precision of the sliders 16, although not essentially necessary, is extremely strict; for example, a to levance of about: 2 .mu.m is required. As described above, the width of the U bar 50 is set to the same as that of the I bar 54 because it is difficult to provide a predetermined difference, such as about 100 .mu.m, between them. If a processing method on surface reference on the BH side described below is not adopted, processing precision of the U bar 50 and the I bar 54, variations depending on deformation of blanks of both bars, and variations in set of glass bonding of both bars are involved and a predetermined difference, such as about 100 .mu.m, cannot be guaranteed. To consider the variations, if the U bar 50 and the I bar 54 are made having different in widths, the difference must be set to a larger value than the predetermined difference. When coils and back cores are incorporated to form a magnetic head in post processing, the larger setup difference affects the contact area of the back core; when the contact area is small, magnetic resistance becomes large, resulting in degradation of the electromagnetic conversion characteristic of the head. Therefore, the U and I bars have been set to the same width.
However, in such a manufacturing method, processing is performed with the surface on the PH side of the U bar 50 as reference as described above, thus the number of steps in processing the surface on the BH side increases and extremely strict height precision of the sliders 16 and extremely strict PH precision of the U bar 50 is required, raising manufacturing costs. On the other hand, GD precision is lowered due to variation in height precision of the sliders 16, and the electro-magnetic conversion characteristic is degraded.
Formerly, track definition groove processing of the record/reproduce core 12 and that of the erase core 14 were performed in separate batches because the track definition grooves 34 and 42a and 42b differ in pitch. In track definition groove processing of the record/reproduce core 12 and that of the erase core 14, as shown in FIG. 5, a core blank 60 of either of the record/reproduce core 12 or the erase core 14 was set on a jig 62 which was set on a work table 64. By moving a blade 66 back and forth in the direction Y and the vertical direction Z and moving the work table 64 in the horizontal direction X, a large number of track definition grooves 34 were cut in the length of the core blank 60R/W of the record/reproduce core 12; a large number of track definition grooves 42a and 42b were cut in the length of the core blank 60E of the erase core 14.
In cutting track definition grooves 34, 42a, and 42b of core blanks 60 of the record/reproduce core 12 and the erase core 14, the relationship between the number of core blanks 60R/W of the record/reproduce cores 12 and a cumulative pitch error is shown as curve R/W in FIG. 6; the relationship between the number of core blanks 60E of the erase cores 14 and a cumulative pitch error is shown as curve E in FIG. 6. The curves R/W and E are not the same because the processing conditions do not match and the processing time is prolonged as described above. As the processing time is prolonged, air and grinding lubricant temperatures of a pneumatic spindle 68 of the dicing saw change and the spindle 68 may therefore expand or contract.
Assume that the feed rate of a core blank (GB bar) 60R/W of the record/reproduce core 12 is 5 mm/sec and that of a core blank 60E of the erase core 14 is 2 mm/sec and that the core blank length is 100 mm. With pitch P as 600 .mu.m, two grooves are cut per pitch for the core blank 60R/W of the record/reproduce core 12. The number of grooves is EQU 100,000.div.600.times.2=333 grooves.fwdarw.330 grooves
Three grooves are cut per pitch for the core blank 60E of the erase core 14. The number of grooves is EQU 100,000.div.600.times.3=499 grooves.fwdarw.495 grooves
Assume that the X axis feed width of the move shaft of the work table 64 which is at right angles to the rotation shaft (spindle 68) of the blade 66 of the dicing saw is 200 mm as shown in FIG. 7. If core blanks 60 are arranged on 4-mm pitches, then 200+4=50 core blanks are arranged. The cutting time per groove of the 50 core blanks 60 requires EQU 200 mm.div.5 mm/sec=40 sec
for the core blanks 60R/W of the record/reproduce cores 12; EQU 200 mm.div.2 mm/sec=100 sec
for the core blanks 60E of the erase cores 14.
Therefore, the total processing time for 50 core blanks 60 (8250 core chips) is EQU 40.times.330=13200 sec=220 min=4 hr
for the core blanks 60R/W of the record/reproduce cores 12; EQU 100.times.495=49500 sec=825 min=14 hr
for the core blanks 60E of the erase cores 14. If the return time of the blade 66 of the dicing saw is contained, the total processing time takes 5 hr for the core blanks 60R/W of the record/reproduce cores 12 and 15 hr for the core blanks 60E of the erase cores 14.
The blade 66 is attached to the pneumatic spindle 68 of the dicing saw, and the spindle 68 air and grinding lubricant temperatures change with time, expanding or contracting the spindle 68.
Assume that the temperature changes 5.degree. C. for 5 to 15 hr, the time taken to process 50 core blanks 60R/W or 60E. If the overhang amount from the Y axis measurement position of the spindle 68 is 200 mm, EQU 200,000 .mu.m.times.5.degree. C..times.100.times.10.sup.-7 (line expansion coefficient)=10 .mu.m
A pitch error occurs even if Y axis pitch feed precision is improved.
If the core blanks 60R/W and 60E are long, they extend over a large number of pitches, thus 1-pitch errors are accumulated, thereby increasing pitch shift as the pitches are placed backward.
As described above, if the track definition grooves 34, 42a, and 42b of the record/reproduce core 12 and the erase core 14 are cut separately, even when the track definition grooves 34 of the record/reproduce core 12 are cut on small pitches, the track definition grooves 42a and 42b of the erase core 14 become large or vice versa.
Therefore, if a combined magnetic head core is manufactured by joining the inner core section 22 of the record/reproduce core 12 and the inner core section 38 of the erase core 14, center line T1 of the record/reproduce core 12 does not match center line T2 of the erase core 14 as shown in FIG. 8, and if a misalignment between the center line T1 of the record/reproduce core 12 and the center line T2 of the erase core 14 exceeds the allowable range of 4 .mu.m, an adjacent track erases a record and the actual recording track width is narrowed.
Since 1-pitch errors are accumulated as described above, track groove processing cannot be performed for a long (30 mm or longer) ferrite core structure, hindering efficient manufacturing of combined magnetic head cores.
Further, a guide to the unformatted recording capacity of a 3.5-inch floppy disk drive is 2M bytes. The magnetic head used for 2M-byte floppy disks is of tunnel erase type. For the erase characteristic of the head, the actual capability of the product is sufficiently superior to the specification. For example, for sweep track erase (STE), the specification is -28 dB while the actual capability is -40 dB.
However, for the read/write (R/W) characteristic, the actual capability of the product is not sufficiently superior to the specification. Particularly for Sidel, for example, for 2F output, the specification is 1.3 mVpp or more while the actual capability is about 1.5 mVpp (nominal); for overwrite (OW), the specification is -28 dB while the actual capability is about -30 dB; for resolution (Res), the specification is 70% or more while the actual capability is about 75%.
Further, the OW and Res specifications must be satisfied for disks manufactured by various manufacturers and the read/write characteristic of the magnetic head must be even as consistent as possible.
The read/write characteristic varies depending on the degree of loss in record/reproduce core 12 and the degree of loss occurring between the core 12 and a disk. To make the magnitude of the former loss even, it is important to make magnetic resistance of the record/reproduce core 12 even.
FIG. 9 is a front view of a core structure for illustrating magnetic resistance of a core made of ferrite, etc. FIG. 10 is a side view of the core structure in FIG. 9. In FIGS. 9 and 10, the magnetomotive force of the core, inductance (L), is inversely proportional to magnetic resistance of the core and proportional to the square of the number of coil turns, N. EQU L=k (1) EQU R=Ri=Rg+Rc+Rbg (2)
where
Rg is magnetic resistance of magnetic gap 30; PA1 Rc is magnetic resistance of core (outer core section 20, inner core section 22, back core section); and PA1 Rbg is magnetic resistance of back gap 30a. PA1 combining a first core member having a predetermined pole height dimension defined by a surface on a magnetic gap formation side and an apex and a predetermined back height dimension defined by the apex and a bottom of a coil core which is inserted into a coil, and a second core member so that a magnetic gap of a predetermined dimension is formed on the pole height side and forming a core blank by a first glass bonding; PA1 forming track definition grooves of a predetermined depth on a surface on the pole height side of the core blank for forming tracks of a predetermined width and forming a read/write core structure and an erase core structure; PA1 combining the read/write and erase core structures and forming a combined core structure by a second glass bonding; and PA1 cutting the combined core structure to a desired thickness to provide combined core chips, PA1 using the bottom of the coil core of the first core member of the core blank as a back height reference face and with the back height reference face as reference, grinding the surface on the pole height side of the core blank; PA1 with the back height reference face of the first core member of the core blank as reference, forming track definition grooves of a predetermined depth on the surface on the pole height side of the core blank for forming tracks of a predetermined width and forming a read/write core structure and an erase core structure; PA1 with the back height reference face of the first core member as reference, combining the read/write and erase core structures and forming a combined core structure by the second glass bonding; and PA1 with the back height reference face of the first core member as reference, grinding the surface on the pole height side of the combined core structure. PA1 combining a first core member having a predetermined pole height dimension defined by a surface on a magnetic gap formation side and an apex and a predetermined back height dimension defined by the apex and a bottom of a coil core which is inserted into a coil, and a second core member so that a magnetic gap of a predetermined dimension is formed on the pole height side and forming a core blank by a first glass bonding; PA1 forming track definition grooves of a predetermined depth on a surface on the pole height side of the core blank for forming tracks of a predetermined width and forming a read/write core structure and an erase core structure; PA1 combining the read/write and erase core structures and forming a combined core structure by a second glass bonding; and PA1 cutting the combined core structure to a desired thickness to provide combined core chips, PA1 using the bottom of the coil core of the first core member of the core blank as a back height reference face and with the back height reference face as reference, grinding the surface on the pole height side of the core blank; PA1 with the back height reference face of the first core member of the core blank as reference, forming track definition grooves of a predetermined depth on the surface on the pole height side of the core blank for forming tracks of a predetermined width and forming a read/write core structure and an erase core structure; PA1 with the back height reference face of the first core member as reference, combining the read/write and erase core structures and forming a combined core structure by the second glass bonding; PA1 with the back height reference face of the first core member as reference, grinding the surface on the pole height side of the combined core structure; and PA1 with the back height reference face of the first core member of the combined core chip as reference, combining the combined core chip and a slider and grinding a top of the slider and a surface on the pole height side of the combined core chip for forming a magnetic gap of the combined core chip to a predetermined gap dimension. PA1 combining a first core member having a predetermined pole height dimension defined by a surface on a magnetic gap formation side and an apex and a predetermined back height dimension defined by the apex and a bottom of a coil core which is inserted into a coil, and a second core member so that a magnetic gap of a predetermined dimension is formed on the pole height side and forming a core blank by a first glass bonding; PA1 forming track definition grooves of a predetermined depth on a surface on the pole height side of the core blank for forming tracks of a predetermined width and forming a read/write core structure and an erase core structure; PA1 combining the read/write and erase core structures and forming a combined core structure by a second glass bonding; and PA1 cutting the combined core structure to a desired thickness to provide combined core chips, PA1 using the bottom of the coil core of the first core member of the core blank as a back height reference face and with the back height reference face as reference, grinding the surface on the pole height side of the core blank; PA1 with the back height reference face of the first core member of the core blank as reference, placing a read/write core blank on one side of a jig and an erase core blank on the other side of the jig so that they are positioned opposite to each other and forming track definition grooves of a predetermined depth on the surface on the pole height side of the two core blanks for forming tracks of a predetermined width and forming the read/write core structure and the erase core structure; PA1 with the back height reference face of the first core member as reference, combining the read/write and erase core structures and forming a combined core structure by the second glass bonding; and PA1 with the back height reference face of the first core member as reference, grinding the surface on the pole height side of the combined core structure. PA1 a read/write core and an erase core each provided by combining a first core member having a predetermined pole height dimension defined by a surface on a magnetic gap formation side and an apex and a predetermined back height dimension defined by the apex and a bottom of a coil core into which a coil is inserted, and a second core member so that a magnetic gap of a predetermined dimension is formed on the pole height side; PA1 the erase core having track definition grooves whose depth from the surface on the pole height side of the erase core shallows gradually from the second core member to the first core member for defining a track of a predetermined width; PA1 the read/write core having track definition grooves whose depth from the surface on the pole height side of the read/write core structure is constant for defining a track of a predetermined width, PA1 wherein the second core members of the read/write and erase cores are located and fixed so that they are opposite to each other.
The magnetic resistance Rc changes with a change in the track definition groove depth. Particularly, the inner core section 22 has large magnetic resistance, thus it is desirable to make the track definition groove depths as shallow and even as possible.
FIG. 11 is a front view of a core blank with dimension indication to calculate the BH dimension and the grooving height.
In this case, since the workpiece touch face of the jig 62, which is the reference face, is inclined, the height precision of the jig 62 is bad, therefore the precision of the C dimension becomes bad; the C dimension must be made large. Thus, the magnetic resistance R becomes large. As compared with horizontal track definition grooves, inclined track definition grooves 34 are cut deeper in the inner core section, thus the magnetic resistance R becomes larger, but the ferrite member of the inner core section 22 is originally thin and the magnetic resistance R is large; it is not preferable to increase the magnetic resistance R.
FIG. 12 is an illustration of a gap disconnection section in a core structure. With the inclined track definition groove 34 as shown in FIG. 12, reflected light does not come into view, thus the gap disconnection section dimension Q is not seen.
With the conventional record/reproduce core 12, only the track definition grooves 34 are filled with glass 46 and other portions of the contact area 20a remain with the ferrite member intact. The ferrite part may be broken off, causing signal read noise to occur. It is necessary to work so as not to break off the ferrite part. This makes it impossible to raise the work speed.
FIGS. 13, 14, and 15 are sectional views showing the state of the contact face taken on lines I-I', II-II', and III-III' of FIG. 1 respectively . At the conventional combined magnetic head core 10, on the section taken on line I-I' of the record/reproduce core 12, an aqueous solution is used for lapping in the manufacturing process. Thus, as shown in FIG. 13, high melting point glass portions a at the left and right of a read/write track 32 in the inner core section 22 of the record/reproduce core 12 are recessed about 0.02 .mu.m and low melting point glass portions b are recessed 0.05 .mu.m. On the section taken on line II-II' of the erase core 14, as shown in FIG. 14, high melting point glass portions c at the left and right of an erase track 44 in the inner core section 38 are recessed about 0.02 .mu.m and low melting point glass portions d are recessed 0.05 .mu.m. Further, on the section taken on line III-III', as shown in FIG. 15, high melting point glass portions e are recessed and the inner core section 22 of the record/reproduce core 12 slopes slightly down to the left, opposite a center shield part f.
Since glass centers around the magnetic gap (read/write gap) 30 and the magnetic gap (erase gap) 40, the glass 46 is stepped and the magnetic gap (read/write gap) 30 is recessed 0.005-0.01 .mu.m below its surrounding, leading to a spacing loss and causing the read/write characteristic to be lowered 10-20%. Since the track definition grooves 34, 42a, and 42b of the record/reproduce core 12 and the erase core 14 are tilt grooves, orientation of the record/reproduce core 12 and the erase core 14 are not visible and they are hard to assemble.